1. Field of the Invention
The invention relates to a process for preparing protective-colloid-stabilized polymers based on vinyl esters and ethylene, optionally with further comonomers, in the form of their aqueous polymer dispersions or water-redispersible polymer powders, by means of continuous emulsion polymerization without employing a seed latex, and when appropriate, drying the resultant polymer dispersion.
2. Background Art
Protective-colloid-stabilized polymers are used, especially in the form of their aqueous dispersions or water-redispersible polymer powders, in a wide variety of applications, for example as coating compositions or adhesives for application to a wide variety of substrates. The protective colloids are generally polyvinyl alcohols. The use of polyvinyl alcohol as a protective colloid is desirable, since this material contributes to strength properties, for example to the tensile bond strengths of tile adhesives, as opposed to systems stabilized by low-molecular-weight compounds (emulsifiers). The monomers preferably used for preparing redispersible powders have hitherto been vinyl esters and ethylene, since it is difficult to stabilize acrylate copolymers or styrene-acrylate copolymers using polyvinyl alcohol.
The dispersions are traditionally prepared by batch emulsion polymerization. This procedure has high flexibility and is therefore preferred in manufacturing plants producing a wide range of products. In addition, this procedure is much less technically demanding than a continuous process. A disadvantage compared with the continuous process is the much lower cost-effectiveness of batch processes.
A continuous process would therefore be a desirable choice for plants with high output of a narrow range of products. The principle problem with continuous emulsion polymerization is establishing suitable conditions for constant nucleation of new latex particles. The favored plant configuration is a stirred tank cascade, since reactors of this type are already used in batch operations and could therefore be used for the continuous process with only low conversion costs. The product properties and the process parameters usually vary to a greater of lesser extent, however, and in extreme cases can be the cause of unusable product or even damage to the plant.
DE-A 2456576 describes a configuration for continuous polymerization composed of a reactor for the main polymerization and a tubular reactor for postpolymerization. The polymerization is carried out in the presence of a redox initiator system, using a large excess of reducing agent. The monomers are metered in such that their concentration does not exceed a defined limit. A disadvantage of this process is the rapid formation of wall deposits, leading to low plant availability. Among the reasons for formation of wall deposits are the marked variations in product properties.
U.S. Pat. No. 4,164,489 describes a process for the continuous preparation of vinyl acetate-ethylene dispersions using a seed latex which is likewise introduced continuously.
In the process of EP-A 1067147, it is recommended that for high-solids-content vinyl acetate-ethylene polymers, polymerization take place in the presence of a seed latex and also a low-molecular-weight polyvinyl alcohol. DE-A 10035587 describes a continuous polymerization process in which the final product can be used as a seed latex, and also a version of the process in which it is possible to work without a seed latex. However, the seed-latex-free process is unsatisfactory with respect to particle size distribution of the resultant products; the particles obtained are excessively coarse.
It is an object of the present invention to provide a process which continuously prepares vinyl ester-ethylene copolymer dispersions which, even without the use of seed latex, provides products without oscillating product properties and with monomodal particle size distribution with weight-average diameter Dw of from 800 to 1400 nm.